Advances in integrated circuit manufacturing have been closely tied to advances in the related arts on which integrated circuit manufacturing depends. One of the fundamental processes used in the manufacture of integrated circuits is optical lithography. Optical lithography uses visible or ultraviolet radiation to selectively expose portions of a photoresist which has been deposited onto a semiconductor wafer. After the photoresist has been selectively exposed, it is developed, to remove either the exposed or unexposed photoresist, thereby forming a bas-relief image on the surface of the wafer. Photoresists that become more soluble in a developer after they are exposed are called positive resists. Photoresists that are less soluble after being exposed are called negative resists. Each area of the image on the wafer may define a component, or feature, of the circuit being fabricated. Features typically include both semiconductor components, such as a transistor gate, and metal components such as interconnects.
The developed photoresist remaining on the wafer forms a mask which protects the surface below the photoresist from subsequent process steps. For example, if a wafer with a photoresist mask is exposed to a dopant, only the portions of the wafer not covered by the mask would receive the dopant. Likewise, if a masked wafer with a metal surface layer is etched, only the unmasked portions of the metal layer are etched away leaving the masked or protected portions to form interconnects between circuit elements.
One of the major benefits from improved lithography processes is the reduction in size of semiconductor circuitry. As the size of the circuitry has been reduced, both the amount of circuitry that can be built on a given substrate and the operating speed of the circuitry have increased. As the amount of circuitry on a substrate and the speed of the circuitry have both increased, the circuitry cost has decreased, increasing economically feasible applications. Thus, the reduction in circuitry size has played a major role in the growth of the semiconductor industry.
As devices become smaller, it is more difficult to produce the lithography masks required to image the circuitry with adequate image resolution and contrast. This is especially true when using lithography techniques, such as contact printing, where the lithography mask is in direct contact with the wafer and must therefore have the same feature size as the target circuitry.
Projection lithography separates the mask from the wafer and uses lenses to focus the light passing through the mask onto the wafer. The lenses between the lithography mask and the wafer may also reduce the size of the projected image of the mask to allow the use of an oversized lithography mask. However, because the lithography mask is not against the wafer, light that is diffracted by one aperture of the lithography mask may interfere with light from another aperture of the lithography mask. The light interferes both constructively to create bright regions where there should be dark regions, and destructively to create dark regions where there should be light regions. This is especially significant at or beyond the Airy limit when the light from two adjacent apertures constructively interferes and exposes the photoresist between the apertures.
To prevent the constructive interference of two adjacent features, a phase shifting mask is sometimes used. The phase shifting mask reduces the problems of constructive interference but has the disadvantage of requiring a more complex lithography mask. Furthermore, the benefits of phase shifting lithography masks are difficult to achieve with some circuitry patterns. For example, a `U` shaped feature having a phase shifter on one vertical segment to avoid constructive interference with the other vertical segment will cause destructive interference to occur on the short horizontal segment where the phase shifter ends, leaving an inadvertently unexposed portion. Therefore, there is a need in the art for an improved optical lithography process that is capable of forming small circuit features.